West Nile Virus in North America: Where are We and What Can We Do About It?

West Nile virus [WNV] is a small-sized RNA virus transmitted by mosquitoes that produces a wide spectrum of clinical symptoms in humans and horses, ranging from no apparent illness in about 80% of infections, to fever in about 20%, to neuroinvasive disease and death in <1%. The frequency of serious illness is greatest in the elderly. The virus originally was discovered during a fever survey in the West Nile District of Uganda in 1937, but its medical importance most likely was masked by the myriad of childhood fevers and malaria, until outbreaks of varying magnitude were described in South Africa, upper Egypt and India in the 1950s, Madagascar in the 1970s, and then the Mediterranean and eastern Europe in the 1990s. A subtype, named Kunjin virus, was discovered in Australia in the 1960s, but only recently has been recognized as a cause of equine disease. In endemic areas, acquired immunity during childhood imparts long lasting protection that protects against more serious neuroinvasive disease usually acquired later in life. This was not apparent until recent 'virgin soil' outbreaks in Eastern Europe and Russia. As with many emerging infections, it has been difficult to separate the sequence of evolution and virus dispersal from clinical recognition and laboratory diagnosis.